@soapbox.pub/wasmboy/core/memory/readTraps.ts

Soapbox fork of Wasmboy.

import { Memory } from './memory';
import { Cpu } from '../cpu/index';
import { Graphics } from '../graphics/graphics';
import { Lcd } from '../graphics/index';
import { batchProcessAudio, SoundRegisterReadTraps, Channel3 } from '../sound/index';
import { eightBitStoreIntoGBMemory } from './store';
import { eightBitLoadFromGBMemory } from './load';
import { Joypad, getJoypadState } from '../joypad/index';
import { Timers } from '../timers/index';
import { Interrupts } from '../interrupts/index';
import { checkBitOnByte, resetBitOnByte, splitHighByte } from '../helpers/index';

// Returns -1 if no trap found, otherwise returns a value that should be fed for the address
export function checkReadTraps(offset: i32): i32 {
  // Cache globals used multiple times for performance
  let videoRamLocation = Memory.videoRamLocation;

  // Try to break early for most common scenario
  if (offset < videoRamLocation) {
    return -1;
  }

  // Check the graphics mode to see if we can read VRAM
  // http://gbdev.gg8.se/wiki/articles/Video_Display#Accessing_VRAM_and_OAM
  if (offset >= videoRamLocation && offset < Memory.cartridgeRamLocation) {
    // Can only read/write from VRAM During Modes 0 - 2
    // See graphics/lcd.ts
    // TODO: This can do more harm than good in a beta emulator,
    // requres precise timing, disabling for now
    // if (Graphics.currentLcdMode > 2) {
    //   return 0xFF;
    // }

    return -1;
  }

  // ECHO Ram, E000	FDFF	Mirror of C000~DDFF (ECHO RAM)
  // http://gbdev.gg8.se/wiki/articles/Memory_Map
  if (offset >= Memory.echoRamLocation && offset < Memory.spriteInformationTableLocation) {
    // Simply return the mirror'd value
    return eightBitLoadFromGBMemory(offset - 0x2000);
  }

  // Check for individal writes
  // Can only read/write from OAM During Modes 0 - 1
  // See graphics/lcd.ts
  if (offset >= Memory.spriteInformationTableLocation && offset <= Memory.spriteInformationTableLocationEnd) {
    // Can only read/write from OAM During Mode 2
    // See graphics/lcd.ts
    // if (Lcd.currentLcdMode < 2) {
    // return 0xff;
    // }

    // Not batch processing here for performance
    // batchProcessGraphics();

    // return -1;
    return Lcd.currentLcdMode < 2 ? 0xff : -1;
  }

  // CPU
  if (offset === Cpu.memoryLocationSpeedSwitch) {
    // TCAGBD, only Bit 7 and 0 are readable, all others are 1
    let response = 0xff;

    let currentSpeedSwitchRegister = eightBitLoadFromGBMemory(Cpu.memoryLocationSpeedSwitch);
    if (!checkBitOnByte(0, currentSpeedSwitchRegister)) {
      response = resetBitOnByte(0, response);
    }

    if (!Cpu.GBCDoubleSpeed) {
      response = resetBitOnByte(7, response);
    }

    return response;
  }

  // Graphics
  // Not batch processing here for performance
  // batchProcessGraphics();
  if (offset === Graphics.memoryLocationScanlineRegister) {
    eightBitStoreIntoGBMemory(offset, Graphics.scanlineRegister);
    return Graphics.scanlineRegister;
  }

  // Sound
  // http://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware#Registers
  // TODO: Put these bounds on the Sound Class
  if (offset >= 0xff10 && offset <= 0xff26) {
    batchProcessAudio();
    return SoundRegisterReadTraps(offset);
  }

  // FF27 - FF2F not used
  // http://gbdev.gg8.se/wiki/articles/Gameboy_sound_hardware#Register_Reading
  // Always read as 0xFF
  if (offset >= 0xff27 && offset <= 0xff2f) {
    return 0xff;
  }

  // Final Wave Table for Channel 3
  if (offset >= 0xff30 && offset <= 0xff3f) {
    batchProcessAudio();

    if (Channel3.isEnabled) {
      return Channel3.handleWaveRamRead();
    }
    return -1;
  }

  // Timers
  if (offset === Timers.memoryLocationDividerRegister) {
    // Divider register in memory is just the upper 8 bits
    // http://gbdev.gg8.se/wiki/articles/Timer_Obscure_Behaviour
    let upperDividerRegisterBits = splitHighByte(Timers.dividerRegister);
    eightBitStoreIntoGBMemory(offset, upperDividerRegisterBits);
    return upperDividerRegisterBits;
  }

  if (offset === Timers.memoryLocationTimerCounter) {
    eightBitStoreIntoGBMemory(offset, Timers.timerCounter);
    return Timers.timerCounter;
  }

  // Interrupts
  if (offset === Interrupts.memoryLocationInterruptRequest) {
    // TCAGB and BGB say the top 5 bits are always 1.
    return 0xe0 | Interrupts.interruptsRequestedValue;
  }

  // Joypad
  if (offset === Joypad.memoryLocationJoypadRegister) {
    return getJoypadState();
  }

  return -1;
}